Inverse latexes of polyelectrolytes based on partially or completely salified 2-methyl-2-[(1-oxo-2-propenyl)amino]-1-propanesulfonic acid (also known as 2-acrylamido-2-methylpropanesulfonic acid, ATBS or AMPS), and their use in cosmetics and/or in pharmaceuticals have formed the subject matter of numerous patent applications. However, the presence of significant amounts of water and oil in these inverse latexes represents a not insignificant disadvantage in terms of volume, of cost and sometimes of increased risks and/or of toxic effects.
Solutions have thus been developed for increasing the concentration of polyelectrolytes in the final inverse latexes, for example by subjecting the reaction medium, at the end of polymerization, to a vacuum distillation stage in order to remove a more or less large portion of water and oil. However, this distillation is problematic to carry out as it often brings about a destabilization of the inverse latex which is necessary to counter by the prior addition of stabilizing agents. European patent applications EP 0 161 038 and EP 0 126 528 and British patent application GB 1 482 515 disclose such a use of stabilizing polymers.
The disadvantage of these stabilizing products is that they comprise alcohols or glycols which can cause environmental problems. Furthermore, sometimes the reaction medium sets solid during the distillation stage, without this phenomenon ever having been truly explained, but the certain consequence of which is the destruction of the batch of inverse latex in the course of preparation and difficult and expensive cleaning of the reactor used. Finally, even if the distillation takes place correctly, the inverse latexes obtained often invert with difficulty when they are employed in an aqueous phase and they also exhibit a high viscosity and sometimes microgels within them. These disadvantages thus prohibit their use in the manufacture of cosmetic formulations and/or of textile printing pastes. In order to overcome these disadvantages, the inventors have developed an inverse latex disclosed in the French patent application published under the number FR 2 879 607 comprising from 50% to 80% by weight of a polyelectrolyte comprising from 0.01 mol % to 10 mol % of at least one monomer unit derived from the compound of formula (A):C(R1)(R3)═C(R2)—C(═O)—O—(CH2—CH2—O)n—R4  (A)in which the R1, R2 and R3 radicals, which are identical or different, represent, independently of one another, a halogen atom or a linear or branched alkyl radical comprising from 1 to 4 carbon atoms, the R4 radical represents a saturated or unsaturated and linear or branched aliphatic radical comprising from 6 to 30 carbon atoms and n represents a number between 1 and 50.
However, when this composition is used to prepare a thickened formulation, the rate of inversion of the inverse latex in the aqueous phase, that is to say the time necessary to obtain the maximum development of the viscosity, remains fairly low, which means, for the user, a loss of time during the use of this product, in particular in the industrial phase for preparing cosmetic formulations and/or textile printing pastes. This is because it is well known that the inversion time for inverse latexes increases very considerably as a function of the scale of use. Furthermore, the stability over time of the inverse latexes described in FR 2 879 607 is not completely satisfactory. This is because a fairly rapid phenomenon of release of oil at the surface is observed during storage.
The inventors have thus sought to develop inverse latexes which do not exhibit the abovementioned disadvantages.